A liquid crystal display device has been widely utilized in a display of a personal computer, a television or the like. The device utilizes physical properties such as optical anisotropy and dielectric anisotropy of a liquid crystal compound. An operating mode of the liquid crystal display device includes such a mode as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a polymer sustained alignment (PSA) mode. In the device having the PSA mode, a liquid crystal composition containing a polymer is used. In the composition, alignment of liquid crystal molecules can be controlled by the polymer.
In such a liquid crystal display device, a liquid crystal composition having suitable physical properties is used. In order to further improve characteristics of the device, the liquid crystal compound contained in the composition preferably has physical properties described in (1) to (8) below. (1) High stability to heat and light, (2) a high clearing point, (3) low minimum temperature of a liquid crystal phase, (4) small viscosity (η), (5) suitable optical anisotropy (Δn), (6) large dielectric anisotropy (Δε), (7) a suitable elastic constant (K), and (8) excellent compatibility with other liquid crystal compounds.
An effect of the physical properties of the liquid crystal compound on the characteristics of the device is as described below. A compound having the high stability to heat and light as described in (1) increases a voltage holding ratio of the device. Therefore, a service life of the device becomes long. A compound having the high clearing point as described in (2) extends a temperature range in which the device can be used. A compound having the low minimum temperature of the liquid crystal phase such as the nematic phase and a smectic phase as described in (3), in particular, a compound having the low minimum temperature of the nematic phase also extends the temperature range in which the device can be used. A compound having the small viscosity as described in (4) shortens a response time of the device.
A compound having the suitable optical anisotropy, namely the compound having large optical anisotropy or small optical anisotropy is required according to design of the device. When the response time is shortened by decreasing a cell gap of the device, a compound having the large optical anisotropy is suitable. A compound having the large dielectric anisotropy as described in (6) decreases a threshold voltage of the device. Thus, an electric power consumption of the device is decreased. On the other hand, a compound having the small dielectric anisotropy shortens the response time of the device by decreasing the viscosity of the composition. The compound extends the temperature range in which the device can be used by increasing the maximum temperature of the nematic phase.
With regard to (7), a compound having a large elastic constant decreases the response time of the device. A compound having a small elastic constant decreases the threshold voltage of the device. Therefore, the suitable elastic constant is required according to characteristics to be desirably improved. A compound having the excellent compatibility with other liquid crystal compounds as described in (8) is preferred. The reason is that the physical properties of the composition are adjusted by mixing liquid crystal compounds having different physical properties.
A variety of liquid crystal compounds each having the large dielectric anisotropy have so far been synthesized. A variety of liquid crystal compounds having the large optical anisotropy have also so far been synthesized because good physical properties that are not found in the conventional compounds are expected for a new compound, or because the new product provides at least two physical properties with a suitable balance in the composition in several cases. Under such circumstances, desire has been expressed for a compound having excellent physical properties and the suitable balance regarding the physical properties (1) to (8) described above.
In order to prepare a liquid crystal compound having an excellent characteristic balance, an attempt has been so far made on forming a molecular terminal into a special structure, and Patent literature No. 1 describes compound (S-1) having a perfluoroallyloxy group. Moreover, Patent literature No. 2 describes compound (S-2) having a 1,1,3,3-tetrafluoroallyloxy group in a similar structure.
